The Ukrainian military has started an operation against pro-Russia activists in the city of Mariupol, southeastern Ukraine, as well as the town of Konstantinovka, according to agencies.

The troops moved into Mariupol and surrounded an administrative building held by anti-government protesters. They told RT that the army is warning them that if they do not leave the building in the coming minutes, they will be fired at and the building will be seized.

“I am in the center of the city, there are a lot of ambulances outside the local administration building, gunfire is being heard, armored vehicles have entered the city and are moving towards the center,” witness Tatyana told RT by phone.

Mikhail Krutko from the self-defense headquarters told Interfax news agency: “Residents are unarmed, they blocked roads in the city center, built up barricades from tires and other things not to let hardware pass. They set tires on fire.”

People on the ground told RT that there are 300-500 anti-Kiev protesters there and they are unarmed. There have so far been no reports of injuries.

“There is no fatalities so far, but I can’t say anything about the number of injured. Right now police have returned from the scene, but people in dark uniforms can be seen in other parts of the city,” another witness named Dmitry told RT.

At the same time, a convoy of Ukrainian armed forces has forced its way through self-defense checkpoints near the Donetsk Region town of Konstantinovka. Shots and alarm can be heard inside the city.

There are reports that at least two people have been killed, RT correspondent Paula Slier says. The TV building has reportedly been seized by the army and all transmissions are currently off-air, she adds.

Ukraine's acting interior minister, Arsen Avakov, has confirmed that a special operation is underway in the town of Konstantinovka, adding that several soldiers have been injured.

National Security Council secretary Andrey Parubiy said Kiev plans to begin military operations in other regions of Ukraine after completing its operations in Slavyansk and Kramatorsk.

Ukraine stepped up its military operations in southeastern Ukraine against pro-autonomy activists. The main opposition strongholds include Kramatorsk, Slavyansk, Lugansk, and others.

Meanwhile, clashes between pro-nationalist radicals and anti-Kiev activists have intensified. On Friday, more than 45 anti-Kiev protesters were killed in Odessa; some died in clashes, while others died from burns or suffocation after being trapped inside the House of Trade Unions building by the ultra-nationalist Right Sector group.